1. Field of the Invention
The present invention relates to a differential planet gear unit for use in reduction gears constituting link driving mechanisms for industrial robots.
2. Description of the Background Art
In multi-link type robots in general, a construction with actuators for each link drive dispersively arranged is employed and each of these robots is provided with reduction gears for reducing the rotational speed of each actuator. In this kind of reduction gear system, a large reduction gear ratio as well as small in size and light in weight is required. In addition, reduction gears having a high torque and power transmission with a high torsion rigidity are also required.
Under these circumstances, a conventional differential planet gear unit 100, as shown in FIGS. 1 and 2, has been used as a reduction gear. In the differential planet gear unit 100, a sun gear 103 is rotated by the rotation of an input shaft 101, and the rotation of the sun gear 103 is transmitted to three planet gears 107 arranged at an equal angular distance so as to engage with both the sun gear 103 and a fixed ring gear 105.
The planet gears 107 rotate around their own axes while they also revolve around the sun gear 103. Each of the planet gears 107 is engaged with the fixed ring gear 105 as well as with a rotary ring gear 109 having a slight difference in the number of teeth relative to that of the fixed ring gear 105, thereby rotating an output shaft 111 through the rotary ring gear 109 secured thereto.
As a result, when each of the planet gears 107 is rotated around its own axis while it is also revolved around the sun gear 103, the rotary ring gear 109 is rotated in accordance with the difference in the number of teeth between the rotary ring gear 109 and the fixed ring gear 105, thereby decreasing the rotational speed of the output shaft 111 against that of the input shaft 101.
Now, in the differential planet gear unit 100, in order to precisely engage each planet gear 107 with the fixed ring gear 105 and the rotary ring gear 109 which are coaxially arranged and have the difference in the number of their teeth, as shown in FIG. 3, the addendum modification coefficient of one ring gear having the lesser number of teeth, for instance, the fixed ring gear 105, is formed larger in its design. As a result, the diameter of the addendum circle 105a of the fixed ring gear 105 is almost equal to that of the addendum circle 109a of the rotary ring gear 109.
Furthermore, the planet gears 107 engaging with both the ring gears 105 and 109 are integrally formed and have a common formation so that the diameter of the addendum circle 107a of the planet gear 107, which contacts with both the fixed and rotary ring gears 105 and 109, is equal to those of the latter.
In such a differential planet gear unit 100 described above, a large reduction gear ratio can be obtained due to the differential action in accordance with the difference between the tooth numbers of the two ring gears 105 and 109. In addition, since the planet gears 107 engaging with both the fixed and rotary ring gears 105 and 109 are integrally formed and have the common formation, as described above, deformation of the planet gears is reduced, and, since the output from the rotary ring gear is directly picked up, the torsion rigidity is improved. As a result, the differential planet gear unit thus integrally formed becomes small in size and light in weight due to the reduction in the number of parts to be used.
On the other hand, the following problems arise in the conventional differential planet gear unit 100. In the other reduction gears for robots of late years, the power transmission efficiency often exceeds more than 80%, whereas in the reduction gear system of this kind having a tooth number difference of three between the fixed and rotary ring gears in the gear unit 100 described above, its power transmission efficiency is approximately 70% to 75%.
Consequently, since the power transmission efficiency remains low although the reduction gears are diminished and are formed in light weight, there is a problem that large size motors are needed in order to compensate for the power loss thereof. The cause of this reduced efficiency in the differential planet gear unit 100 has long been unsolved.
The inventor of this application has found the reason or cause of the low power transmission efficiency after conducting various experiments for the differential planet gear unit.
The efficiency of the differential planet gear unit 100 can be sought from the ratio of contact of the gears in the unit 100. Particularly, each of the ratios of contact between the fixed ring gear 105 and the planet gear 107 and between the rotary ring gear 109 and the planet gear 107 greatly effects the power transmission efficiency.
FIG. 4(a) shows the involute tooth forms of the fixed ring gear 105 and the planet gear 107 and a contact action line l.sub.1 thereof in the gear unit 100. Since the addendum modification coefficient of the fixed ring gear 105 is very large as compared with that of the planet gear 107, a contact pitch point Pc leans largely toward the addendum from the center of the tooth height of the planet gear 107. As a result, it is to be appreciated that an approaching contact length e.sub.1, i.e., a straight line segment A - pc between a contact start point A and the contact pitch point Pc is short while a receding contact length e.sub.2, i.e., a straight line: segment Pc - B between the contact pitch point Pc and a contact end point B is much longer.
Similarly, FIG. 4(b) shows the involute tooth forms of the rotary ring gear 109 and the planet gear 107 and a contact action line l.sub.2 thereof. In this contact, a contact pitch point Pd leans largely toward the addendum from the center of the tooth height of the planet gear 107. Consequently, it is appreciated that the approaching contact length e.sub.1, i.e., A - pd is short whereas the receding contact length e.sub.2, i.e., Pd - B is much longer.
As is well-known, in involute gears having a general involute tooth form, when the teeth are engaged with each other at the contact pitch point, their efficiency become high as a result of 100% rolling. However, when the teeth are engaged with each other at a point off from the pitch point, the sliding velocity increases in proportion to the offset distance from the pitch point while reducing its efficiency. In the engagement of profile shifted gears, on the other hard, since the contact pitch point is largely offset from the center of the tooth height of the gears, either the approaching contact length or the receding contact length will become extremely long, thereby increasing the sliding velocity while decreasing the efficiency of the gear unit. This causes the reduction of the power transmission efficiency in the conventional differential planet gear unit.